In word processing systems it is often necessary to revise a given document one or more times. Normally it becomes difficult to determine what parts of the document have been changed. A line by line or word by word comparison between an old version of the document and an edited updated version of the document is often necessary.
It is desirable to be able to analyze at a revised document and to tell therefrom what changes (insertions and/or deletions) have been made since the previous version.
In previous systems, there was no or very little way to determine changes that had been made in a revised document. Text that had been deleted, for example, did not appear either on a display screen or on a printed document. Previous methods for indicating these changes in text have been to annotate in the margin of the document areas that have been changed within the document. These attempts at solving the problem have not been entirely successful, due to the fact that old and new versions of the document must still be compared line by line or word by word to determine whether correct changes have been made or whether any insertions or deletions have been made at all.
It would be desirable not to have to make a word by word comparison between old and new versions. Changes on a document should be shown on the same document so that no comparison between versions of a document is required.